1. Field of the Invention
This invention relates in general to means and methods for low noise semiconductor junctions and devices and, more particularly, to improved means and methods for low noise zener diodes having improved characteristics.
2. Background Art
Voltage reference diodes are well known in the semiconductor art. These devices are designed to go into non-destructive Zener or avalanche breakdown at particular voltages, typically from about 1.8 volts to more than 200 volts. As used herein, the term "Zener" or "Zener diode" is intended to encompass all such voltage reference behavior or devices, irrespective of the particular breakdown mechanism which gives rise to the voltage reference behavior.
In general, Zener diodes consist of a p-n junction formed from two oppositely doped semiconductor regions having carefully controlled dopant concentration and distribution. For Zener diodes with minimum breakdown voltage, i.e. about 1.8 volts, the p and n regions must both be very highly doped, typically in the range 10.sup.19 to 10.sup.21 per cm.sup.3, and the junction must be abrupt. This requires a very steep doping profile. In the prior art, alloying has been a preferred method of fabricating Zener diodes in the voltage range of 1.8 to about 10 volts.
The Zener voltage may be increased by reducing the doping, first on one side of the p-n junction, for voltages in the range 1.8 to about 10 volts, and then on both sides of the p-n junction for voltages approximately in the range 10 to 200 volts. Diffusion has been a preferred method for fabricating Zener diodes which operate in the range of about 10 to 200 volts. Diodes having voltages around 10 volts may be fabricated by either method.
Despite great effort at process control there is still considerable variation in the properties of Zener diodes which nominally have received identical processing. For example, the manufacturing process produces a distribution of Zener voltages, dynamic impedances, and noise output rather than a single Zener voltage, dynamic impedance, and noise output. As a consequence, the manufactured product must be sorted to obtain the particular voltage, dynamic impedance, and noise characteristics desired. This is less economical since the distribution of manufacturing output does not in general match the distribution of market demand. To obtain an adequate supply of a particularly popular set of Zener characteristics, other devices having less popular Zener characteristics must inevitably be manufactured. Sometimes these must be discarded. Accordingly, there is a continuing need for improved means and methods for Zener diodes having more predictable, controllable, and repeatable manufacturing yield to a desired target specification.
A particular problem with prior art devices is the noise figure. Zener diodes generate electrical noise which limits their usefulness in many circuit applications. This noise is believed to arise from, among other things, minute fluctuations in the breakdown process. Some noise is inevitable in all devices, but the typical noise levels of prior art Zener diodes has been higher than has been desired. While some prior art Zener diodes can exhibit relatively low noise, just as some prior art diodes can have Zener voltages in a particular range, there has not existed in the prior art a method for manufacturing a high yield of Zener diodes with consistent and uniform low noise characteristics.
Accordingly, it is an object of the present invention to provide improved means and methods for Zener diodes having improved and closely grouped characteristics.
It is a further object of the present invention to provide improved means and methods for Zener diodes having lower noise than generally obtained with the prior art.
It is an additional object of the present invention to provide improved means and methods for Zener diodes having equal or lower dynamic impedance for a given Zener voltage than generally obtained with the prior art.
It is a still further object of the present invention to provide improved means and methods for Zener diodes wherein a greater proportion of the manufacturing output has lower noise and/or dynamic impedance than obtainable with the prior art.
It is an additional object of the present invention to provide improved means and methods for Zener diodes wherein the above objectives are obtained simultaneously.